As the mercury soars in many parts of Australia, and heads into the 40's, heat stress in cattle is back on the agenda.

A spate of feedlot deaths in the late 1990's and early 2000's focussed attention on the industry, and new stock handling standards were brought into play.

President of the Australian Veterinary Association Dr Ben Gardiner says that cattle producers and feedlot operators are now much more acutely aware of heat stress.

"Excessive heat stress is where cattle are not able to dissipate body heat effectively and their body temperature rises above normal.

"Heat initially leads to reduced feed intake and production losses but can, in extreme cases, lead to tissue organ damage and death.

"The factors that contribute to body heat load in cattle are complex and include environmental conditions and animal characteristics.

"Cattle in feedlots are more susceptible to heat stress because of the smaller area where cattle are housed.

"Some of the factors that can contribute to heart stress include recent rainfall, a high ongoing minimum and maximum ambient temperature, high humidity, an absence of cloud cover with a high solar radiation level, minimal air movement over an extended period of 4 or 5 days or a sudden change in climate.

"Some cattle are more susceptible to heat stress than others.

"Bos Indicus cattle like Brahman's are more heat tolerant than Bos Taurus cattle like Angus. Cattle with lighter coat colour tend to be more tolerant of heat and heavier cattle tend to be more susceptible to the heat.

"Feedlot operators can minimise the heat load burden placed on animals during hot conditions by implementing a range of management strategies like increasing ventilation, ensuring the cattle have access to shade, water and that conditions are not too cramped," Dr Ben Gardiner says.

The deadly combination of high temperatures, high humidity and lack of wind was blamed for the deaths of 1250 cattle in the Australia Meat Holdings Prime City Feedlot near Griffith in Southern NSW in March 2000. The Lot Feeders Association at the time said the deaths were due to an unfortunate combination of climatic conditions, high rainfall, high humidity and high temperatures. It said water was available to the cattle and a third of the feed lot was under shade.

The worst heat-related feedlot incident was the death of 2680 head at the Whyalla feedlot near Texas, Queensland in February, 1991.